A typical power converter may employ a closed loop controller to regulate its output voltage within specified bounds. Such a power converter generally employs a closed-loop current mode controller, as shown by FIG. 1, in which an output voltage is sampled, compared to a stable reference voltage to develop an error signal, and processes this error signal to produce a current command that is sent to the power stage of the converter, which converts this command into an output current. The processing of the error signal has traditionally been performed by analog circuitry in the frequency domain, and is designed so that the power converter exhibits a desired closed loop bandwidth that will meet the required dynamic response characteristic to load current changes. This processing may also be performed by firmware running in a microcontroller; the output voltage is conditioned by analog circuits and then sampled at a fixed rate and converted to a digital value that is compared to a reference value to develop an error value. This error value is processed by a proportional-integral-differential (PID) controller to produce the current command, which is converted back to an analog signal for use by the power stage as previously described. The control loop in this instance does not benefit from the differential portion of the PID controller, therefore it is omitted in subsequent references, and herein the controller will be referred to as a proportional-integral (PI) controller. The digital processing offers the ability to incorporate additional features that may be impractical to implement in analog circuitry.
FIG. 1 is a diagram showing the functions performed in a typical prior art control loop. The output voltage of the power converter, in general, may be scaled to match the input voltage range of the small-signal circuits that are included in the PI controller (proportional-integral controller); this scaled signal is labeled Vout SCALED in FIG. 1. In an analog implementation, the functions may be implemented using, for example, a stable voltage reference microcircuit, operational amplifiers and discrete resistors and capacitors, in a manner that is well known. In a digital implementation, Vout SCALED may be converted to a digital value by an analog-to-digital converter and the data in the controller are sampled and processed at a fixed rate that is typically 10 to 100 times the desired loop bandwidth. This value is subtracted from a reference value by a difference (or subtractor) 110 to produce an ERROR value.
The ERROR value is provided as input to a first multiplier 121 that multiplies the ERROR value by a constant Ki, that feeds an integrator 130, and the ERROR value is also provided to a second (proportional) multiplier 122 that multiplies the ERROR value by a constant Kp, to provide the proportional gain function. The integrator 130 accumulates successive values of its input. The output of the integrator 130 and the output of the proportional multiplier 122 are summed by summer 140, producing the current command value as output, which may be subsequently converted to an analog voltage to control an output current of a power stage (not shown) that converts the current command to the output voltage Vout. The bandwidth and phase margin of the closed control loop may be governed by the selection of the constants Kp and Ki.
Previous power supplies have often had to compromise between providing rapid transient response while limiting input current modulation. Therefore, there is a need in the industry to address one or more of the abovementioned shortcomings.